152 METHODS OF MICEOSCOPICAL RESEARCH. 



and whilst it is hot to, very rapidly, replace a form 

 in its desired position, whilst with a fine needle all 

 hairs and other imperfections can be removed and 

 absolute cleanliness ensured. 



About a quarter of a century since, when slides 

 of "selected" and "grouped" diatoms, &c., were 

 in the zenith of their popularity, and finding, from 

 painful experience and repeated failures, that the 

 successful mounting of these and other such pre- 

 parations was surrounded by almost insuperable 

 difficulties, in respect both of affixing the siliceous 

 valves to the thin covers on which they were to be 

 placed, and the selecting perfect forms from the 

 debris and refuse of the cleaned diatomaceous 

 material, and safely transferring them to the slide 

 in course of preparation, the author, in order to 

 conquer these difficulties, devised a cement (for 

 which the formula will be given in its place), and 

 which has proved equally safe, permanent and 

 invisible, even under the highest powers of the 

 microscope, and, indeed, the only really reliable 

 method of fixing diatomaceae, spiculse, polycystina, 

 &c. It is the only cement, also, so far as the 

 author knows, which has stood the test of many 

 years, all other cements used for this purpose (and 

 all of which he has tried) having sooner or later, 

 and generally sooner rather than later, succumbed 

 t o the disintegrating action of the Canada balsam, 

 or other media in which the forms have been 

 mounted, with the result that they have become 

 detached from the cover and valuable and expen- 

 sive slides thus destroyed and rendered worthless. 

 Finding also by equally annoying experience that 

 the so-called " mechanical finger " and other such 



